void dfs(Graph g)
{
   int i;
   visited = calloc(nV(g),sizeof(int));
   order = 1;
   while (order < nV(g)) {
      Vertex v;
      for (v = 0; v < nV(g); v++)
         if (visited[v] == -1) break;
      dfsR(g, v);
   }
}